[Patent document 1] JP-A No.202168/2002
[Patent document 2] Japanese Patent No. 3366818
Generally known air flowmeters which measure the intake air flow rate in an internal combustion engine of a car or the like include: an air flowmeter which measures the intake air flow rate according to heat radiation of a heating element (heating resistor) to be controlled; and a thermal air flowmeter which measures the flow rate according to the difference between the upstream and downstream temperatures of a heating element. The latter is called a temperature difference type thermal air flowmeter.
In a temperature difference type thermal air flowmeter, electrical insulating film (supporting film) is laid over the surface (top face) of a flat substrate (semiconductor substrate like a silicon substrate) and a heating element (heating resistor) is provided on this insulating film and temperature detectors (resistance temperature detectors, thermocouples, etc) are provided on both sides (air flow upstream and downstream) of the heating element. The flat substrate is coated with protective film in a way that the heating element and temperature detectors are covered.
For the purpose of thermal insulation from the heating element, a cavity is made in the flat substrate by edging to create a thermal insulating space. In the prior art, this thermal insulating space is almost square as described in the above patent documents 1 and 2.
In the prior art, the thermal insulating area on the flat substrate is almost square. The thermal insulator of the thermal insulating area on the flat substrate consists of thin electrical insulating film (diaphragm) with a thickness of several micrometers. Because the thermal insulating area is almost square, in consideration of strength, the size (square measure) of the thermal insulator is limited. If the air contains particles of dust, the particles will collide the thermal insulator and destroy the thin film (several micrometers in thickness). Since the size of the thermal insulating area is limited, it is impossible to improve the air flow rate measurement sensitivity, reduce power consumption of the heating element and broaden the measuring range.